For direct fuel injection in internal combustion engines, typical injectors include a piezo drive or a solenoid drive. Solenoid injectors are formed predominantly as multi-hole injectors with an inwardly opening nozzle needle, whereas piezo injectors often have a conical jet and an outwardly opening nozzle needle.
Piezo injectors are generally of highly complex construction because a stable injection function is demanded in a pressure range from for example 5 bar to 200 bar. To meet this demand, a so-called pressure-balanced nozzle needle may be employed. This leads to an increase in costs. A major advantage of piezo injectors with an outwardly opening nozzle needle consists in the mixture preparation quality. This makes it possible, even in the case of a considerably lowered injection pressure such as for example 100 bar, for legal requirements with regard to admissible particle count to be satisfied in a stable manner throughout the service life.
In relation to piezo injectors, solenoid injectors are of simpler and less expensive construction. In particular, no idle travel compensation is necessary. Furthermore, solenoid injectors do not have pressure-balanced nozzle needles. Owing to the inwardly opening movement direction, in the case of which a very small needle seat diameter of for example 1.4 mm can be realized, this is not as disadvantageous as in the case of injectors with outwardly opening nozzle needles, which typically have seat diameters of approximately 3.6 mm to 4 mm. However, in the case of high injection pressures of, for example, 350 bar to 500 bar, increasing difficulties are encountered even in the case of solenoid injectors. A particular challenge in the case of solenoid injectors with multi-hole nozzles is that of satisfying legal requirements with regard to particle count throughout the entire service life. Various test results have shown that a pressure increase from 200 bar to 350 to 500 bar reduces the particle count significantly and in a stable manner throughout the service life. On the other hand, the costs of the injection system are however thus also greatly increased.